1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method and a storage medium. In particular, the present invention relates to an image processing technique for selecting an image suitable for use in diagnosis and surgery from a plurality of X-ray digital subtraction angiography images.
2. Description of the Related Art
Recent advancement in digital technology has made it common to apply digital processing to images in the medical field. In place of the conventional X-ray imaging that uses films for X-ray diagnosis, two-dimensional X-ray sensors that output X-ray images as digital images have become widespread. The application of digital image processing to digital images output from the two-dimensional X-ray sensors comes in a broad range.
One example of suitable application of such digital image processing is digital subtraction angiography (DSA) processing for acquiring a DSA image. A DSA image is a subtraction image acquired as follows. Images are acquired before and after a radiopaque dye is injected into an object. The result of subtracting the image acquired before the injection of the radiopaque dye (hereinafter, “mask image”) from the image acquired after the injection of the radiopaque dye (hereinafter, “live image”) is a subtraction image. A DSA image not only keeps a blood vessel region, which is a region of interest used in diagnosis, as a region exhibiting a difference between the images acquired before and after the injection of the radiopaque dye, but also removes all unnecessary region other than the region exhibiting the difference as a background region and shows the background region in a uniform fashion. That is to say, in a DSA image, a region of blood vessels filled with the radiopaque dye, which becomes less visible in a live image due to obstruction by the background region, clearly appears as a subtraction image. Such a DSA image is commonly used in vascular interventional radiology (IVR). Vascular IVR is a technique to insert a catheter into a target organ in a body while checking X-ray fluoroscopy images.
Regarding DSA processing, for example, Japanese Patent Laid-Open No. 1-221142 discloses the invention of an X-ray image processing apparatus suitable for acquiring orientation images that make it easy to understand how the blood vessels are arranged during surgery and the like.
Normally, X-ray fluoroscopy images cannot show blood vessels with high X-ray transmittance. To show such blood vessels, a radiopaque dye is administered. However, in vascular IVR, if the radiopaque dye is injected continuously until a catheter reaches a target organ, then a large dosage of radiopaque dye would be ultimately administered, which increases the burden on a patient. In view of this, in general vascular IVR, a predetermined dosage of radiopaque dye is administered prior to fluoroscopy, and images are captured successively thereafter. By subtracting a pre-angiography image from successive post-angiography images, subtraction images are acquired as moving images that serve as DSA moving images. From the DSA moving images, a frame that clearly shows blood vessels through which the catheter should be advanced is selected (hereinafter, “reference image”). The reference image is displayed on a second monitor that is different from a main monitor for displaying fluoroscopic images. The reference image is used as a guide to advance the catheter during fluoroscopy.
In general vascular IVR, the following operations are carried out repeatedly until the catheter reaches a target organ: a catheter operation via fluoroscopy, successive imaging under administration of a radiopaque dye, generation of DSA moving images from acquired images, and selection of a frame used as a reference image. The administered dosage of radiopaque dye can be suppressed by using a reference image acquired prior to fluoroscopy instead of operating the catheter under administration of the radiopaque dye during fluoroscopy.
Conventionally, in order to select a reference image from DSA moving images, a technician searches for an optimal frame through visual examination by, for example, reproducing, frame-forwarding and frame-reversing the DSA moving images. In general vascular IVR, the catheter does not reach a target organ by a catheter operation involving a single generation of DSA moving images and a single fluoroscopy. That is to say, generation of DSA moving images and selection of a reference image are repeated multiple times in accordance with the state of catheter insertion. Therefore, selection of a reference image is a very troublesome operation for the technician and could increase the amount of time required for the treatment.
In view of the above problems, the present invention provides an image processing technique that enables reduction in the effort required to select a reference image to be displayed on a second monitor during vascular IVR.